Development of a solid oxide fuel cell, having a laminate structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer (oxidant electrode layer) and a fuel electrode layer, is progressing as a third-generation fuel cell for use in electric power generation. In a solid oxide fuel cell, oxygen (air) is supplied to an air electrode section and a fuel gas (H2, CO and the like) is supplied to a fuel electrode section. An air electrode and a fuel electrode are both made to be porous so that gases can reach interfaces in contact with the solid electrolyte layer.
Oxygen supplied to an air electrode section passes through pores in the air electrode layer and reaches a neighborhood of the interface in contact with the solid electrolyte layer, and in that portion, the oxygen receives electrons from the air electrode to be ionized into oxide ions (O2−). These generated oxide ions move in the solid electrolyte layer by diffusion toward the fuel electrode. The oxide ions having reached the neighborhood of the interface in contact with the fuel electrode react with fuel gas in that portion to produce reaction products (H2O, CO2 and the like), and release electrons to the fuel electrode.
The electrode reaction when hydrogen is used as fuel is as follows:
Air electrode: 1/2O2+2e−→O2−
Fuel electrode: H2+O2−→H2O+2e−
Overall: H2+1/2O2→H2O
Because the solid electrolyte layer is a medium for migration of the oxide ions and also functions as a partition wall for preventing direct contact of the fuel gas with air, the solid electrolyte layer has a dense structure capable of blocking gas permeation. It is required that the solid electrolyte layer has high oxide ion conductivity, and is chemically stable and strong against thermal shock under conditions involving an oxidative atmosphere in the air electrode section and a reductive atmosphere in a fuel electrode section. As a material which can meet such requirements, generally a stabilized zirconia (YSZ) that is added with yttria is used.
On the other hand, the air electrode (cathode) layer and fuel electrode (anode) layer need to be formed of materials having high electronic conductivity. Because an air electrode material is required to be chemically stable in an oxidative atmosphere of high temperatures around 700° C., metals are unsuitable for the air electrode, and generally used are perovskite type oxide materials having electronic conductivity, specifically LaMnO3 or LaCoO3, or solid solutions in which part of an La component in these materials is replaced with Sr, Ca and the like. Moreover, the fuel electrode material is generally a metal such as Ni or Co, or a cermet such as Ni—YSZ or Co—YSZ.
A solid oxide fuel cell is classified into a high temperature operation type operated at high temperatures around 1000° C., and a low temperature operation type operated at low temperatures around 700° C. A solid oxide fuel cell of the low temperature operation type uses an electric power generation cell which is improved to work as a fuel cell even at low temperatures by lowering a resistance of an electrolyte, for example, through making the electrolyte made of an yttria stabilized zirconia (YSZ), be a thin film on the order of 10 μm in thickness.
A solid oxide fuel cell operable at high temperature is used for the separator, for example, a ceramic having electronic conductivity such as lanthanum chromite (LaCrO3), while a solid oxide fuel cell of a low temperature operation type can be used for the separator, i.e. a metallic material such as stainless steel.
Additionally, as structure of the solid oxide fuel cell, there have been proposed three types, namely, a cylindrical type, a monolithic type and a flat plate type.
A stack of a solid oxide fuel cell has a structure in which electric power generation cells, current collectors and separators are alternately laminated. A pair of separators sandwich an electric power generation cell from both sides of the cell in such a way that one of the separators is in contact with the air electrode through intermediary of an air electrode current collector, while the other separator is in contact with the fuel electrode through intermediary of a fuel electrode current collector. For the fuel electrode current collector, a spongy porous substance made of a Ni based alloy or the like can be used, while also for the air electrode current collector, a spongy porous substance made of a Ag based alloy or the like can be used. A spongy porous substance simultaneously displays a current collection function, gas permeation function, uniform gas diffusion function, cushion function, thermal expansion difference absorption function and the like, and is accordingly suitable for a multifunction current collector.
The separators electrically connect between electric power generation cells, and also have a function to supply gas to the electric power generation cells. Therefore, each separator has a fuel path through which fuel gas is introduced from a peripheral side of the separator and is discharged from a separator surface facing the fuel electrode layer, and an oxidant path through which oxidant gas is introduced from the peripheral side of the separator and is discharged from a separator surface facing the oxidant electrode layer.